Faraday Discussions最新文献

We will discuss how sustained nonequilibrium processes operating at the plasma membrane (PM) determine the dynamical organisation (both lateral and transverse) of lipids, their maintenance and control, under physiological conditions. These nonequilibrium processes include active contractile stresses arising from the inevitable interaction of the inner leaflet of the PM with the adjoining actomyosin cortex, and active flipping of specific lipids. Recently, we showed that the inner leaflet phosphatidylserine (PS) interacts with the actomyosin cortex and engages in a strong transbilayer coupling across the leaflets. Here we develop an active Flory–Huggins theory for the mesoscale segregation of liquid-ordered (lo)–liquid-disordered (ld) domains in an asymmetric membrane bilayer, that incorporates both active contractile stresses at the inner leaflet and transbilayer coupling across the leaflets. The interplay between chemical potential gradients, transbilayer coupling and active stresses drives a rich pattern of mesoscale lo domains – static, strongly fluctuating and moving active emulsions – even at temperatures beyond the equilibrium phase transition temperature. We study conditions under which domain registry and slippage could be observed. We end with a discussion on the role of active flippases on PS in maintaining the active mesoscale organisation.

The asymmetry between the two leaflets of a plasma membrane (PM) is widely accepted as an essential condition for most PM-associated biochemical processes. However, recent work has also shown that asymmetric bilayers can be significantly stiffer upon bending than symmetric ones, suggesting that the same asymmetry may hinder the ability of the PM to remodel itself. Here, we address this issue by combining all-atom molecular dynamics (MD) simulations with an enhanced sampling scheme that explicitly induces membrane deformations to quantify their free-energy cost. Examining small asymmetric POPC/DOPC bilayers, we find that a small density imbalance between the leaflets increases their bending rigidity compared to bilayers with minimal imbalance, or to symmetric bilayers of the same two lipids. This result is consistent with recently proposed theoretical models that identify differential stress as the main source of stiffening in asymmetric membranes. The first-principles approach used in this study is broadly applicable to other types of membrane, enabling further exploration of the interplay between asymmetry and curvature, or the simulation of specific biological conditions of the PM.

Polylactide (PLA) is a commercial and sustainably sourced aliphatic polyester but its applications have been limited by its low toughness. The insertion of a rubbery segment within the PLA backbone is among the promising strategies to enhance the mechanical properties of PLA while retaining sustainability. Herein, we disclose a catalytic stereoretentive ring-opening metathesis polymerization process to access high molar mass (M^exp_n up to 127.9 kg mol⁻¹) all-cis telechelic polycyclooctene (PCOE) at low catalyst loadings. The use of cis-1,4-diacetoxy-2-butene as a chain-transfer agent in the presence of stereoretentive dithiolate Ru carbenes afforded precise control over the cis content, the molar mass, and the introduction of acetoxy chain ends. Subsequent hydrolysis of the acetoxy motifs followed by chain extension via ring-opening polymerization of D,L-lactide yielded high molar mass (M^exp_n up to 105.0 kg mol⁻¹) all-cis PLA ABA triblock copolymers. The influence of the molar mass of the all-cis PCOE over the thermal and mechanical properties of the ABA triblock was investigated.

Biomembranes show asymmetric lipid composition of their two leaflets. The phenomenon that ordered domains in one leaflet may affect the order of the other has been referred to as interleaflet coupling and discussed in terms of transmembrane signaling. Many coupling mechanisms have been proposed; one potential mechanism should arise from the fact that the isolated melting of an ordered, e.g., gel phase gives rise to a significant expansion of this leaflet, resulting in a mismatch between the intrinsic areas of the leaflets. This asymmetry stress can be accommodated in a number of ways. One is interleaflet coupling – individually higher- and lower-melting leaflets melt together at intermediate melting temperature. Alternatively, the membrane may bend towards the larger-intrinsic-area leaflet, bud and release very small daughter vesicles (DVs). Here, we prepared lipid-asymmetric large unilamellar vesicles (aLUVs) with low-melting stearyl-oleyl-phosphatidylcholine (SOPC) in the inner and SOPC with ∼20 mol% of high-melting dipalmitoyl phosphatidylglycerol (DPPG) in the outer leaflet. Phase transitions in aLUVs versus LUVs were recorded with pressure perturbation calorimetry; vesicle budding was monitored by asymmetric flow field-flow fractionation (AF4) and visualized by cryo-transmission electron microscopy. An HPLC protocol was established to quantify the total DPPG content; zeta potential was used to detect outer-leaflet DPPG. It turned out to be possible to prepare aLUVs at 5 and 15 °C, with the outer leaflet partially in the gel phase. The properties of the final aLUVs depended on the preparation temperature. aLUVs prepared at 5 and 15 °C caused the budding of exovesicles upon heating and only weak coupling of the phase transitions of the leaflets. aLUVs prepared at 30 °C with both leaflets in the fluid state showed stronger coupling upon asymmetric freezing/melting at lower temperature. This is in line with the hypotheses that (i) the exchange of lipid between close-to lipid-saturated cyclodextrin and acceptor vesicles at a given temperature results in largely stress-free bilayers and (ii) that outside budding and coupling are, to some extent, alternative responses of the bilayer to asymmetric expansion. These hypotheses help explaining our and some literature data; the overall understanding and prediction of coupling for any given aLUV system remains an urgent, open question.

Advances in X-ray nanoprobe beamlines at synchrotrons across the world present exciting opportunities for rich multimodal imaging of biomineral structures and their formation processes. The combination of techniques provides a sensitive probe of both chemistry and structure, making X-ray nanoprobes an important tool for investigating crystallite growth and orientations, interfaces, and assembly of building blocks into hierarchical structures. A discussion of these capabilities is presented with reference to recent examples using a range of nanoprobe imaging techniques for investigating enamel structure, as well as coccolith properties. Key opportunities for the use of X-ray nanoprobes lie in exploiting the penetrating power and coherence properties of synchrotron X-rays in order to image in situ processes or apply coherent diffractive imaging techniques to obtain higher resolutions. To this end initial results demonstrating the observation of calcium phosphate mineralisation, in a liquid environment, using nano-X-ray fluorescence mapping are presented, and the role of X-ray dose and beam induced effects is considered. Finally novel results from tomographic ptychography imaging of Mytilus edulis mussel shell calcite prisms are discussed, where the segmentation of the phase density into organic and mineral content gives insights into the mechanisms underlying mineral prism formation and the role of the organic matrix in biomineralisation.

The substantial diversity in phospholipids within a plasma membrane, varying in tail length, degree of saturation, and head-group functionality, generates widespread structural heterogeneity. This exists both laterally across the membrane through the spontaneous formation of condensed domains that differ from their surrounding expanded phase in density, composition, and molecular packing order, as well as between its two leaflets, which normally maintain significant compositional asymmetry. Of particular importance is the exposure of phosphatidylserine (PS) lipids which is a marker for important physiological processes e.g. apoptosis. Despite this, the molecular-level alterations to the phase-structure of the membrane that result from PS exposure remain generally unknown. In this work, we utilise recently developed phase-resolved azimuthal-scanned sum-frequency generation (SFG) microscopy to investigate structural changes that occur heterogeneously across model membranes as a result of PS-lipid exposure. Specifically, by probing mixed monolayers of 1,2-dipalmitoylphosphatidylcholine (DPPC) and deuterated 1-palmitoyl-2-oleoylphosphatidylcholine (dPOPC) in both the C–H and C–D stretching regions as well as equivalent films with DPPC exchanged with DPPS, we analyse the variations in the apparent phase distributions and domain morphologies, and quantitatively extract the density, composition, and relative out-of-plane packing order for both mixtures. We find that, in these mixtures, DPPS shows vast differences in the domain growth and coalescence behaviour compared to DPPC, as well as in the relative compositions and molecular ordering within each phase. This demonstrates the critical role the head-group plays in the heterogeneous phase structure of the membrane and may give insights into their impact on important physiological processes.

The precise synthesis of multifunctional block copolymers with tailored architectures remains a pivotal challenge in polymer chemistry, particularly when balancing chemoselectivity and stereoselectivity within a single catalytic system. To address this challenge, we report the dual chemoselective and stereoselective capabilities of a commercially available chiral thiourea catalyst, (S,S)-TUC, for the synthesis of well-defined block copolymers. By leveraging its dual selectivity, (S,S)-TUC enables distinct polymerization pathways dictated by monomer composition. In the TMC/rac-LA system, stereoselective ring-opening polymerization (ROP) of rac-LA preferentially consumes D-LA to form PDLA blocks, followed by simultaneous ROP of TMC and L-LA, yielding pentablock copolymers. Conversely, in the PA/PO/rac-LA system, alternating copolymerization of PA and PO precedes stereoselective ROP of rac-LA, generating pentablock architectures. Comprehensive characterization (NMR, SEC, in situ IR, CD spectroscopy) confirms the catalyst’s dual selectivity and adaptability. Notably, (S,S)-TUC operates under mild conditions, eliminates the need for multiple catalysts, and offers cost-effectiveness and low environmental toxicity. This work establishes a unified platform for synthesizing structurally complex copolymers, bridging the gap between precision polymerization and sustainable manufacturing. The methodology holds promise for applications in biodegradable materials, high-performance composites, and biomedical devices, where tailored polymer properties are critical.

Polyhydroxyalkanoates (PHAs) hold significant potential as sustainable alternatives to fossil-based plastics because of their bio-based origin and inherent biodegradability. Poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) is a well-known commercial member of the PHA family characterized by good mechanical resistance and thermal behavior similar to that of some conventional polymers, such as polypropylene. However, its high crystallinity and fragility limit its application. Poly3-hydroxybutyrate-co-4-hydroxybutyrate (P(3HB-co-4HB)) is a new commercial copolymer containing a 4-hydroxybutyrate (4HB) segment that provides increased flexibility because of its amorphous phase. In this study, PHBV and P(3HB-co-4HB) were blended by extrusion, varying the percentage of P(3HB-co-4HB) to improve the PHBV properties without losing the PHA assets and potentializing the insertion of this biopolymer in the market. The results indicate that the impact energy required for fracture was increased in the polymer blends. These blends exhibited greater thermal stability than pure PHBV, with no significant changes observed in the melting and crystallization temperatures. Furthermore, blending was found to reduce shrinkage in injection-molded samples. The degradation in the soil increased with the highest P(3HB-co-4HB) content. Through 3D printing, it was observed that the blends led to an increase in the melt flow index and a reduction in warpage in the printed objects, thereby facilitating the processing of these materials. Consequently, incorporating P(3HB-co-4HB) into PHBV has emerged as a promising strategy to address the inherent limitations of PHBV. This approach not only enhances the mechanical properties and thermal stability but also improves the overall processability, thereby expanding the potential applications of this biopolymer blend.

The fenestrated ultrastructure of the sea urchin endoskeleton has attracted the attention of researchers in different fields due to its morphological complexity and crystallographic properties. Microscopic calcitic trabeculae form an intricate bicontinuous network, called the stereom. The stereom exhibits a wide variation of pore patterns, but is essentially a single calcite crystal (mono-crystalline). The polymorphism and crystal orientation in the skeletons of sea urchins have both been previously extensively described, mostly for taxonomical reasons and for mechanical studies. Moreover, while the resemblance of the stereom architecture to constant-mean-curvature (CMC) structures has been pointed out, a quantitative description and critical analysis is still lacking. Here, we use synchrotron micro-computed tomography to capture the three-dimensional (3D) architecture of the skeletal stereom in sea urchins for morphological quantification. By characterising the different stereom types, we define a data processing pipeline that allows inter-individual and interspecies comparison of stereom architectures, with implications for sea urchin taxonomy, mechanics, and skeletal growth. We further show that the various stereom morphologies are bicontinuous CMC surfaces that are unconstrained by crystallography. Our results highlight the properties of the soft tissue filling the stereom pore space in defining the shape of sea urchin biocalcite.

Atmospheric chemistry in cold environments refers to key chemical processes occurring in Earth’s atmosphere in locations relevant for society including the polar areas, the free and upper troposphere, and the stratosphere. Atmospheric chemistry in these areas is relevant for local air quality, ecosystem health, regional and global climate. This Faraday Discussion comprised excellent coverage of these areas in terms of longitude and latitude, altitude and temperature. It also featured a broad coverage of disciplines between physical, analytical and theoretical chemistry and also the related fields covering aspects of biology, health, meteorology, social sciences and even including policy and economic aspects. A core aspect of the discussions was rooted in interfacing the related diverse competences. Because traditional atmospheric chemistry has evolved around knowledge of mechanisms and kinetics of chemical reactions first in the gas phase and later including condensed phases of aerosol particles and ground surfaces centering around room temperature, the speciality of relevance in this Faraday Discussion was the recent progress in better understanding the evolution of multiphase chemistry at low temperatures, where many relevant properties such as solubility and volatility change dramatically. This was embedded in discussions of the results and challenges of the most recent measurements from a range of campaigns and long-term observations at research stations. The discussion evolved around the chemical cycles of important trace constituents, the formation and evolution of particulate matter under cold conditions, the link between cloud glaciation and air-mass characteristics, air-quality in cold urban environments, biosphere–atmosphere interactions in a warming Arctic, but also the role of interfacial chemistry and reactivity as they are involved in multiphase chemistry processes. Future threats for the cold part of our atmosphere come from increasing human activities in both polar regions with their impacts on ecosystems, air quality and broader scale atmospheric composition as well as from discussions of geoengineering via solar radiation modification by stratospheric aerosol injection.